Engine controllers are known which control ignition timing for various reasons. For example, U.S. Pat. No. 4,144,853 discloses a control system in which engine ignition timing is retarded during cold operation to more rapidly warm the catalytic converter. Idle speed control systems are also known which advance or retard ignition timing to rapidly increase or decrease idle speed in response to an idle speed error signal. U.S. Pat. No. 5,303,168 discloses such a system.
The inventors herein have recognized numerous problems with the above approaches. One such problem is that retard of ignition timing and the resulting decrease in engine idle speed may result in a decrease in intake manifold vacuum below a desired vacuum level. Such a problem is more likely to occur during cold engine operation when: ignition timing is retarded for catalyst warmup; ignition timing may be further retarded to control engine idle speed; and engine friction is relatively high.
Another problem recognized by the inventors occurs during idle speed control when air flow inducted into the engine through a bypass throttle valve positioned in parallel to the main throttle. Control of the bypass throttle valve is limited to a preselected range of authority. When the bypass throttle valve is open to its maximum range of authority, it is unable to increase engine speed to compensate for a retard in ignition timing. An undesired decrease in idle speed may then result.